Abstract: In order to weld with high efficiency and good quality,it is necessary to sense the welding process accurately. It is of great importance to monitor multi-signal in plasma arc welding process. Therefore,plasma arc welding system with sensing multi-signal was established,which is consist of plasma arc welding power and plasma arc welding torch,mechanical execution units,visual sensing equipment,welding process monitor equipment,and computer controlling units. The plasma arc is captured by visual sensing equipment,and ratio between arc diameter at the nozzle and arc diameter at the work-piece is measured. The ratio value is 0. 64,which confirms that the arc in plasma arc welding is high stiffness and low divergence. The thermal profile of weld is monitored by the thermal profile scanner in welding process monitor equipment. It is found that the thermal profile of weld can reflect the real weld in plasma arc welding very precisely.